That's a less efficient protocol than 9pfs and virtiofs, even if you subtract the encryption.
An example of improving efficiency: virtiofs has a relatively recent feature to map pages from host memory directly into guest memory, but that's a lot of risky acrobatics if your priorities are reliability and isolation...
... but it's not supported by Virtualization Framework's built-in virtiofs "folder sharing". (sad face)
... but someone could build it on top of the new macos 27+ custom virtio device support. (intrigued face)
My suggestion: Don't use the host filesystem from the guest at all. It'll be faster, and better isolated. It's a false convenience.